Feature/callback messages (#220)

application.go

@@ -104,7 +104,7 @@ func (a *Application) Run() error {
 	glfw.WindowHint(glfw.ContextVersionMinor, 1)
 	glfw.WindowHint(glfw.OpenGLProfile, glfw.OpenGLCoreProfile)
 	glfw.WindowHint(glfw.OpenGLForwardCompatible, glfw.True)
-  
+
 	if a.config.windowInitialLocations.xpos != 0 {
 		// To create the window at a specific position, make it initially invisible
 		// using the Visible window hint, set its position and then show it.

embedder/embedder.go

@@ -3,13 +3,18 @@ package embedder
 // #include "embedder.h"
 // FlutterEngineResult runFlutter(void *user_data, FlutterEngine *engine, FlutterProjectArgs * Args,
 //						 const char *const * vmArgs, int nVmAgrs);
+// FlutterEngineResult
+// createMessageResponseHandle(FlutterEngine engine, void *user_data,
+//                             FlutterPlatformMessageResponseHandle **reply);
 // char** makeCharArray(int size);
 // void setArrayString(char **a, char *s, int n);
 // const int32_t kFlutterSemanticsNodeIdBatchEnd = -1;
 // const int32_t kFlutterSemanticsCustomActionIdBatchEnd = -1;
 import "C"
 import (
+	"errors"
 	"fmt"
+	"runtime"
 	"runtime/debug"
 	"sync"
 	"unsafe"
@@ -256,8 +261,11 @@ type PlatformMessage struct {
 	Channel string
 	Message []byte
 
-	// ResponseHandle is only set when receiving a platform message.
-	// https://github.com/flutter/flutter/issues/18852
+	// ResponseHandle is set on some recieved platform message. All
+	// PlatformMessage recieved with this attribute must send a response with
+	// `SendPlatformMessageResponse`.
+	// ResponseHandle can also be created from the embedder side when a
+	// platform(golang) message needs native callback.
 	ResponseHandle PlatformMessageResponseHandle
 }
 
@@ -357,8 +365,42 @@ func (flu *FlutterEngine) MarkExternalTextureFrameAvailable(textureID int64) Res
 	return (Result)(res)
 }
 
-// FlutterEngineGetCurrentTime gets the current time in nanoseconds from the
-// clock used by the flutter engine.
+// DataCallback is a function called when a PlatformMessage response send back
+// to the embedder.
+type DataCallback func(binaryReply []byte)
+
+// CreatePlatformMessageResponseHandle creates a platform message response
+// handle that allows the embedder to set a native callback for a response to a
+// message.
+// Must be collected via `ReleasePlatformMessageResponseHandle` after the call
+// to `SendPlatformMessage`.
+func (flu *FlutterEngine) CreatePlatformMessageResponseHandle(callback DataCallback) (PlatformMessageResponseHandle, error) {
+	var responseHandle *C.FlutterPlatformMessageResponseHandle
+
+	callbackPointer := uintptr(unsafe.Pointer(&callback))
+	defer func() {
+		runtime.KeepAlive(callbackPointer)
+	}()
+
+	res := C.createMessageResponseHandle(flu.Engine, unsafe.Pointer(&callbackPointer), &responseHandle)
+	if (Result)(res) != ResultSuccess {
+		return 0, errors.New("failed to create a response handle")
+	}
+	return PlatformMessageResponseHandle(unsafe.Pointer(responseHandle)), nil
+}
+
+// ReleasePlatformMessageResponseHandle collects a platform message response
+// handle.
+func (flu *FlutterEngine) ReleasePlatformMessageResponseHandle(responseHandle PlatformMessageResponseHandle) {
+	cResponseHandle := (*C.FlutterPlatformMessageResponseHandle)(unsafe.Pointer(responseHandle))
+	res := C.FlutterPlatformMessageReleaseResponseHandle(flu.Engine, cResponseHandle)
+	if (Result)(res) != ResultSuccess {
+		fmt.Printf("go-flutter: failed to collect platform response message handle")
+	}
+}
+
+// FlutterEngineGetCurrentTime gets the current time in nanoseconds from the clock used by the flutter
+// engine.
 func FlutterEngineGetCurrentTime() uint64 {
 	return uint64(C.FlutterEngineGetCurrentTime())
 }